New modelling of the U-O-Zr phase diagram in the hyper-stoichiometric region and consequences for the fuel rod liquefaction in oxidising conditions

Liquidus and solidus temperatures were recently re-measured in the UO2+x composition domain by [D. Manara, C. Ronchi, M. Sheindlin, M. Lewis, M. Brykin, J. Nucl. Mater. 342 (2005) 148]. The main difference with the Latta and Fryxell's data [R.E. Latta, R.E. Fryxell, J. Nucl. Mater. 35 (1970) 195] data is that the Manara's transition temperatures were accurately determined using a self-crucible technique while the former data were obtained in a W crucible and then suspected of crucible contamination. According to these recent data, a new thermodynamic modelling of U-O phase diagram is here presented and introduced in the European NUCLEA thermodynamic database for corium applications. An important consequence of this new optimisation for safety applications is that a liquid phase may appear in the O-UO2-ZrO2 composition domain of the U-O-Zr phase diagram at 2600 K at atmospheric pressure (this temperature decreasing with increase of pressure, about 2500 K at 2 atm.). These temperatures can be associated with the temperature at which the fuel assembly could loose its integrity in oxidising conditions and then with what was observed in some of the VERCORS tests where fuel collapse was detected in the temperature range of 2400-2600 K (and quite differently from reducing test conditions) or in the PHEBUS tests where indications of early fuel collapse at 2500-2600 K were identified.